I start a new one marked as question...

What is the sense of the Mono/Stereo switch?

I use my Vetta II HD with an Engl 412SS cab. I just use one amp of the Vetta without any stereo effects. So I decided to use it like my 2204 as a mono guitar amp. => Left/Mono jack and Vetta set to mono. But additional I have to set the Amp Panning of the used amp to "full left" th get the full volume. When I set it to middle, the sound is lower, Amp Panning to right causes NO sound.

This is the behaviour  I expected of using the Stereo setting, just using the left output jack.

From the manual:

24. Left/Mono: These two jacks are wired in parallel and output the left signal (if
you’re running in stereo) or the mono signal if you’re running mono. Be sure
you’ve chosen the corresponding stereo or mono operation on the Vetta front
panel as described in page 4•35.

Mono/Stereo
This page also lets you choose to operate your Vetta in mono or stereo. This is generally
most valuable on the Vetta HD in case you’re using a mono setup, but feel free to
mono-ize your combo if that’s what makes you happy.

Changing Amp Panning
Whether you’re working with a single amp, or two at once, you have complete
control over where they’re placed in the stereo field. You may want to have your
two amps panned hard left and hard right when they’re both on (which gets really
exciting when you’re using separate left and right cabs with Vetta HD, or the Vetta
combo with an external cab in Split Stereo Mode as described on page 4•43), but
then when you kick your FBV switch to turn on only one you may want that amp to
be panned center. To edit your amps’ panning, press the Amp/Cab Details button.
Use the Page knob to select Page 2, ‘Amp Panning’:
AMP1 MIX sets the pan of Amp 1 when it and Amp 2 are both on. AMP1 ONLY
sets pan when only Amp 1 is on. Same idea for the AMP2 controls. Press Amp/Cab
Details again when you’re done, and this display will be dismissed.